1. Field of the Invention
This invention relates to an improved alkylation process for the production of premium grade (high octane number) gasoline by the petroleum industry. More particularly, this invention relates to an improved alkylation process which increases the octane rating of the product alkylate and simultaneously prolongs the catalytic effectiveness of the sulfuric acid by periodic fortification of the alkylation acid and incorporation of a holding time to delay the return of the fortified acid to the alkylation zone.
2. Description of the Prior Art
It is common practice in petroleum refineries to treat low boiling isoparaffins or alkanes with alkylating reagents such as olefins, in the presence of a sulfuric acid catalyst to produce a high octane alkylate, boiling in the gasoline range. It is well known that the sulfuric acid catalyst does not undergo major chemical change during the alkylation process but the acid concentration diminishes due to the build-up of water and organic impurities or red oils originating from undesired side reactions. As the build-up of these diluents approaches 10-12%, the concentration of the alkylation acid is reduced from its original value of about 98.0-99.5% to about 88-90%. At these lower concentration values, the catalytic activity of the alkylation acid is decreased and the octane number of the product alkylate is undesirably lower. At this point the alkylation acid must be withdrawn from the system and reprocessed by regeneration. In reprocessing, the spent acid is usually thermally decomposed to sulfur dioxide, carbon dioxide and water. After purification, the sulfur dioxide generated is reconverted to sulfuric acid by the conventional contact process.
Besides the fuel and energy required for the acid regeneration, another obvious disadvantage of this widely used destructive regeneration process is that the spent acid, consisting mostly (88-90%), of sulfuric acid, is completely decomposed to eliminate the relatively small amounts of water (3-5% by weight) and organic impurities (3-8% by weight). Analyses of spent or used alkylation acid vary somewhat depending on the operating conditions.
In an attempt to overcome these difficulties and extend the effective service life of the alkylation acid, numerous investigations have been directed to understanding the operating variables of the alkylation process which affect the alkylate quality. The importance of a high acid concentration and the maintenance of a constant water content in the alkylation acid is disclosed by U.S. Pat. No. 2,242,845. In the specifications of this U.S. patent, it has been proposed that more concentrated sulfuric acid, for example 98-103% H.sub.2 SO.sub.4, be used as the make-up acid to maintain the strength of the alkylation acid between a concentration of about from 88.0% to 95% and preferably to 91% by weight H.sub.2 SO.sub.4. Alternately, it is disclosed that partially spent acid may be withdrawn from the system and the above concentration range be maintained by fortification with SO.sub.3 or fuming sulfuric acid.
U.S. Pat. No. 2,437,091 discloses that free SO.sub.3, which rapidly attacks the hydrocarbons in the alkylation acid, and water, which is built-up in the system by side reactions during the alkylation, are deleterious both to the alkylate quality as measured by the octane number (O.N.) and the effective service life time of alkylation acid. This patent, U.S. Pat. No. 2,437,091, teaches that part of the alkylation acid be continuously discharged and the balance prechilled and thence continuously fortified with fuming H.sub.2 SO.sub.4 or SO.sub.3 to maintain a water content in the system below 4% by weight and preferably below 1% by weight.
In U.S. Pat. No. 2,465,049, SO.sub.3 or fuming H.sub.2 SO.sub.4 is continuously applied to the alkylation acid and the hydrocarbon feedstocks in a mixer immediately before the alkylation zone to dehydrate the feeds and to convert the water content therein to additional H.sub.2 SO.sub.4 catalyst. A slight excess of a SO.sub.3 -bearing fortifying agent, over that required to react with the water in the hydrocarbon feeds, is applied to remove the water generated by side reactions within the alkylation zone.
In spite of these investigations of the alkylation process, it is still recognized by the petroleum industry that a practical fortification process is needed.
It is an object of the present invention to provide a process for maximizing the octane rating of alkylate product and thereby effectively improving the alkylate yield.
it is another object of this invention to provide a process for reducing the rates of water and organic impurities built-up in the alkylation acid.
It is a further object of this invention to provide a process for fortifying the alkylation catalyst with SO.sub.3 -bearing agents under conditions which minimize the exposure of the fortifying agents to the organic matter dissolved in the acid catalyst.
Still another object of this invention is the extension of the effective service life time of the acid catalyst and the reduction in the quantity of spent acid subject to destructive regeneration.